1. Field of the Invention
The present invention relates, in general, to the remediation of contaminated soil and groundwater. It relates to an in situ method and apparatus to deliver chemical reagents that serve to degrade and/or enhance recovery of organic contamination in soil and groundwater.
2. History of the Related Art
Contamination of subsurface soil and groundwater from the release of hazardous substances has become a significant problem in populated and industrialized areas of the world. The contamination stems from intentional or unintentional releases of hazardous substances from above- and below-ground storage vessels such as tanks and conveyance piping, materials handling practices such as degreasing and other cleaning operations, and other product transfer operations. The released products that are in themselves hazardous or contain hazardous constituents include: petroleum products such as gasoline, diesel fuel, heating oil, jet fuel, and a variety of lubricants; halogenated solvents such as perchloroethylene, trichloroethylene, and freons; and non-halogenated solvents such as hexane, benzene, and ether.
The releases of these substances to the ground surface or into the subsurface presents a risk of human exposure that is unacceptable for human health and ecological factors. Many of the contaminants are known carcinogens and excessive exposure to them has been shown to increase the probability of contracting certain diseases including various forms of cancer. Potential human exposure routes include: inhalation of the volatile contaminant species which can occur as the contaminants in soil and groundwater evaporate into breathing zones; incidental ingestion of contaminated soil; and ingestion of or dermal contact with contaminated groundwater from a potable drinking water well. In response to these concerns, federal, state, and local governments have enacted environmental legislation requiring property owners to investigate and remediate their properties that contain contaminated soil and/or groundwater.
Many techniques have been developed and used over the past two decades to remove or destroy contaminants in soils and groundwater. Early forms of remediation included excavation of contaminated soil for landfill disposal and extraction of contaminated groundwater from wells, above ground treatment followed by discharge to a sewer system or reapplication at the subsurface. Because these methods generally involved substantial financial expenditure and were mostly ineffective, more advanced, in situ methods and techniques of treatment have been developed. These in situ methods include bioremediation, surfactant flushing, and chemical oxidation.
Bioremediation techniques rely on the stimulation of naturally-occurring subsurface bacteria or augmentation with foreign bacteria that, in turn, metabolize the subsurface organic contamination thereby reducing the concentration of the contaminant in soils and groundwater. The effectiveness of in situ bioremediation techniques relies on the uniform and comprehensive delivery of reagents such as inorganic nutrients, oxygen, and bacteria to the region of the subsurface targeted for treatment. Typically, reagent delivery has been accomplished through wells, irrigation trenches, injection lances, or french drains, all of which are established by prior art. Through numerous applications of bioremediation techniques at contaminated sites, it has been shown that these means to deliver reagents do not provide an effective system to supply the necessary reagent flow. Further, each well can reach only a limited volume of soil and groundwater, and, therefore, a large number of the wells, trenches, etc., must be emplaced at a site in order to address an entire contaminated soil or groundwater volume.
Surfactant flushing techniques rely on the delivery of a reagent to the subsurface that serves to reduce the surface tension of organic contaminants which are adhered to soil particle surfaces (commonly referred to as xe2x80x9cadsorbed contaminationxe2x80x9d) or trapped in the interstitial spaces between soil particles (commonly referred to as xe2x80x9cabsorbed contaminationxe2x80x9d). When the delivered chemical reagent (surfactant) contacts the absorbed or adsorbed contaminants (collectively referred to as xe2x80x9csorbed contaminantsxe2x80x9d) the sorption forces are reduced, thereby increasing the mobility of the contaminant which, in turn, increases its ability to be collected and extracted by groundwater extraction (pumping) techniques. Once again, effective application of this technique relies heavily on the delivery device since the surfactant reagent must contact all portions of the soil and groundwater volume that contains the organic contaminant. As with other in situ techniques, surfactant flushing relies on conventional chemical reagent delivery devices such as wells, trenches, injection lances, and french drains to effect emplacement of the surfactant solution. Experience with these devices has again demonstrated that their ability to effect reagent delivery to any significant volume of soil or groundwater surrounding the device is limited since they rely solely on gravitational forces, which may be enhanced by pressurizing the reagent fluid. These apparatus do not promote mixing and turbulence needed to adequately disperse the reagent fluid.
Similarly, chemical oxidation techniques using hydrogen peroxide, Fenton""s Reagent (a combination of hydrogen peroxide, acid, and metal salts) as discussed in Hawley""s Condensed Chemical Dictionary, 11th Edition, Van Nostrand Reinhold, Publishers, 1987, potassium permanganate, sodium permanganate, and ozone rely on the delivery of reagents to the subsurface using wells, trenches, injection lances, or french drains. In addition to their limited effectiveness in comprehensively and economically delivering the required reagents, the use of these conventional delivery techniques can result in dangerous pressure buildup within these devices because of the pressure and heat generated by the reactions that result from chemical oxidation of the contaminants by the supplied oxidation reagents. Generally, a substantial quantity of the oxidizing fluid, in relatively high concentrations (15% to 50% by weight in aqueous form) is needed to effect the desired decomposition of the organic contaminants. Experience with the application of these quantities and concentrations of oxidizing fluids has shown that undesired consequences occur. For example, without adequate dispersement of the reagents from the application apparatus into the surrounding soil and groundwater matrix, the oxidizing fluid can: (1) react with itself, producing large quantities of oxygen gas, which, in combination with the vaporized organic constituent, produce dangerous, explosive conditions; (2) lead to inefficient use of the oxidizing solution; and (3) can cause runaway overpressurization of the apparatus leading to sudden blowout of the apparatus, its components, and/or soil and groundwater surrounding the apparatus.
The present invention relates to an apparatus to effectively deliver chemical reagents to the subterranean environment in a uniform, comprehensive manner, to enhance mixing and lateral and vertical dispersion of the reagents to the soil and groundwater surrounding the apparatus. The chemical reagents delivered by the apparatus include those capable of oxidizing, degrading, or enhancing solubility and mobility organic contaminants sorbed to soils or dissolved in groundwater. The method and apparatus which comprises the present invention accomplishes remediation of soil and groundwater contaminated with undesirable organic compounds by overcoming deficiencies in the art with respect to the delivery of the reagents needed to bring about oxidation, degradation, or solubility and mobility enhancement.
The invention method includes the installation of a plurality of recirculating reagent delivery devices or wells into the subsurface, injection of select chemical reagents into each of the recirculating wells, and recirculation of the reagent/groundwater mixture between the recirculating reagent delivery device and the surrounding soil/groundwater matrix to enhance distribution and mixing of the reagent/groundwater mixture. The apparatus associated with the present invention is the recirculating reagent delivery device. The device consists of: a borehole extending vertically into the subterranean strata; a cylindrical casing centered within the borehole and extending from the ground surface to near the bottom of the borehole; a perforated or screened (permeable) section of the cylindrical casing located near the bottom of the casing to allow reagents to flow outward from the casing or allow groundwater to flow into the casing; a second permeable section of the cylindrical casing located above the lower permeable section to allow either groundwater to flow into the casing or allow supplied reagents to flow out of the casing into the surrounding groundwater body; a reagent delivery string located within the casing and extending to either the upper or lower permeable sections of the cylindrical casing; and reagent mixing, storage, and supply equipment located at the ground surface and attached to the reagent delivery string.
There are two forms of the device in order to address organic contaminants that are denser than groundwater, known as dense, non-aqueous-phase liquids (DNAPL), and organic contaminants that are less dense than groundwater, known as light, non-aqueous-phase liquids (LNAPL). The design of each form of the device (the forced mode and the lift mode, respectively) allows for delivery of the reagents to the region of the groundwater body that is likely to contain a greater concentration or mass of the contaminant. Specifically, the forced mode form of the device is designed to deliver reagents to a greater depth while receiving groundwater for recirculation from a shallow depth because DNAPL contaminants tend to sink, or migrate downward, to greater depths of the groundwater body. Conversely, the lift mode of the device, constructed and operated to address LNAPL contaminants, delivers reagents to the upper portion of the groundwater body, while receiving groundwater from the deeper portion of the groundwater body for recirculation.
The forced mode form of the device is designed and constructed such that the upper and lower permeable sections of the casing are separated by a pump. Groundwater enters the casing through the upper permeable section and is forced by the pump through the lower permeable section and into the groundwater body surrounding the lower permeable section. Reagents are added to the groundwater at the discharge end of the pump through a reagent feed string so that they, in addition to the groundwater, are delivered to the groundwater body surrounding the lower permeable section of the casing.
The lift form of the device is designed and constructed such that the reagent feed string extends to the top of the lower permeable section. Reagents, such as potassium permanganate, Fenton""s Reagent, hydrogen peroxide, ozone, surfactants, or biological nutrients, are pumped into the reagent delivery string, along with a transport medium such as water or compressed air, and emerge in the cylindrical casing, react with dissolved organic contaminants in the groundwater causing gas to form (or, in the case of surfactant, nutrient, or ozone reagents, the supplied air) lifting the fluid in the casing to the upper screened or permeable area, causing the fluid/gas mixture to emerge from the upper portion of the casing and into the surrounding soil/groundwater matrix. The loss of the fluid from the casing allows additional groundwater to flow into the lower screened or permeable section establishing the circulation process which continues as long as reagents are added through the string. The downward motion of the fluid emerging from the upper screen, coupled with the inward motion of groundwater being drawn into the lower screen, creates a torroidal circulation pattern which serves to enhance mixing, reagent distribution, and sorbed contaminant dissolution.
The chemical reactions associated with the use of oxidation reagents, in particular, can generate heat and unwanted vapor containing volatile species of organic contaminants. In cases when this vapor generation is likely (when volatile organic species are present in the groundwater), a vacuum can be applied to the cylindrical casing by a vacuum pump so that the vapor can be evacuated and either discharged to the atmosphere or can be treated with conventional vapor treatment techniques (carbon adsorption, catalytic oxidation, and the like) prior to atmospheric discharge. The applied vacuum also serves to reduce pressures within the freeboard region of the cylindrical casing and within the soils surrounding the device above the groundwater table. The pressure reduction reduces the danger of eruptions of soils and delivery system components that have been experienced by others practicing in situ chemical oxidation procedures using wells, injection lances, trenches, and french drains.